ARM: mvebu: add gpio-keys and input event support in defconfig

Some Armada 370/XP boards have GPIO buttons, so it makes sense to
enable gpio-keys and the input event interface in mvebu_defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>